1. Field of the Invention
The present invention relates to a gauge for determining coating thicknesses on substrates, and, in particular, to a gauge for a combination coating thickness gauge for determining the thickness of a nonferrous coating on a ferrous substrate, and the thickness of a nonconductive coating on a conductive nonferrous substrate.
2. Background Art
In the past, operators of coating thickness gauges were require to know the substrate type before measuring the coating thickness on that substrate. Based on that knowledge, the operator would then choose an appropriate probe configuration and make the appropriate measurement. Typically, an operator would require two independent gauging systems, or two independent probes that could be connect to a single gauge. In addition, the operator would have to determine the characteristics of the specimen (coating and substrate combination) prior to measuring the coating thickness on the determine substrate.
Here is a need for instrument that is able to determine automatically substrate characteristics and measure the coating thickness on that substrate. Such an instrument would enable an operator to place the single gauge probe tip of the instrument on the specimen and, with appropriate instrumentation connected to the gauge probe, to determine the substrate the and automatically measure the coating thickness.
Several dual purpose electromagnetic thickness gauges are known:
For example, U.S. Pat. No.3,986,105 to Nix et al. discloses a probe that uses two coils wound around an elongated ferromagnetic core. One coil is used to carry an excitation current and the other coil is used to make measurements. By altering the frequency of excitation, it is possible to observe the effects of both ferrous and nonferrous conductive substrates on the measurement coil. Low frequency (below 300 Hz) excitation currents allow for the measurement of the thickness of a nonferrous layer on a ferrous substrate, while high frequency (above 1000 Hz) excitation currents allow for the measurement of the thickness of a nonconductive layer on a nonferrous conductive substrate using eddy current effects on the measuring coil. The Nix et at. dual purpose electromagnetic thickness gauge does not utilize a constant magnetic field supplied by a permanent magnet in conjunction with a Hall effect magnetic sensor to measure a coating thickness, nor any means to compensate the measured magnetic flux density for changes in temperature, nor an eddy current search coil excited at a frequency sufficiently high to reduce the depth of penetration of eddy currents into the conductive substrate and thus does not provide a measurement that is sufficiently immune to substrate thickness variations. Furthermore, the Nix et at. dual purpose electromagnetic thickness gauge uses manual, not automatic, switching of thickness gauge modes (low or high frequency of coil excitation) depending on the magnetic character of the substrate.
U.S. Pat. No. 4,005,359 to Smoot discloses an electronic thickness gauge for measuring the coating thickness over ferrous and over conductive nonferrous substrates by measuring the movement of a low-frequency hump in the response curve for an over-coupled, double-tuned, open-faced transformer when ferrous and/or conductive nonferrous substrates are placed across the gap between the transformer pole-faces. Both the coating thickness and magnetic character of the substrate can be determined in one operation without ambiguity, and without any change in the instrument, due to the ability of the device to vary its overall frequency response in accordance with changes in its self-inductance and mutual-inductance. Because the Smoot coating thickness gauge requires the use of an open-faced transformer to measure a coating thickness over ferrous and over conductive nonferrous substrates, and requires the coated substrates to be placed across the relatively large gap between the transformer pole faces, the Smoot device necessarily makes contact with the coated substrate in at least two places, and the area of contact between the Smoot device and the coated surface is therefore relatively large. Since the Smoot coating thickness gauge does not use a Hall generator or an eddy current search coil to conduct either of the coating thickness measurements, the Smoot device cannot be compactly fit into a single gauge probe that contacts the coated surface in at most one place with a much smaller area of contact. There is thus a need for a compact coating thickness gauge probe that tests for a ferrous substrate, using a Hall generator, and tests for a nonferrous substrate, using an eddy current search coil, switching automatically from one to the other in order to measure a coating thickness over ferrous and over conductive nonferrous substrates, the gauge probe contacting the surface of the coated substrate in at most one place with a relatively small area of contact.
U.S. Pat. No. 4,255,709 to Zatsepin et at. discloses a device for measuring the thickness of coatings which, depending on a specific application, employs a different type of thickness gauge, such as electromagnetic, microwave, and ultrasonic, for example. The device measures the thickness of dielectric coatings applied onto metal articles and nonmagnetic electrodeposits on ferromagnetic articles. With an appropriately selected thickness gauge, the device measures the thickness of nonmagnetic, current-carrying coatings on nonmagnetic, current-carrying materials. The Zatsepin et at. coating thickness gauge system employs an automatic measurement range switching unit, but the automatic measurement range switching is used to automatically switch from one thickness measurement range to another without regard to the magnetic character of the underlying substrate. The Zatsepin et at. coating thickness gauge system uses manual, not automatic, switching of thickness gauges depending on the magnetic character of the substrate. There is thus a need for a single gauge probe for a combination coating thickness gauge that enables automatic switching of coating thickness gauging modes depending on an automatic determination of the nature of the underlying substrate.
U.S. Pat. No. 4,722,142 to Schmidt discloses an apparatus for measuring the thickness of a coating that is not highly conductive, such as plastic, on the inside wall of a metallic tubular member, and throughout the length of the metallic tubular member. Preferably, the apparatus uses an electronic eddy current loss proximity detector to measure the coating thickness.
U.S. Pat. No. 5,015,950 to Rose et at. discloses a system for nondestructive analysis of barrier coatings on electrically conductive materials by imposing a controlled known heat load to a localized area of the coating and measuring the changes in the electrical conductivity of the underlying material using an eddy current coil. The electrical conductivity of the material is affected by changes in the temperature related to thermal conduction into the material through the coating, allowing the thickness of the coating, its thermal resistance, and/or its structural integrity to be analyzed, for any type of coating.
Both the Schmidt and Rose et al. coating thickness gauge systems use eddy current detectors to measure coating thicknesses over conductive substrates, but neither the Schmidt nor the Rose et al. coating thickness gauge system uses a Hall generator to measure coating thicknesses over ferrous substrates, and hence neither the Schmidt nor the Rose et al. coating thickness gauge system uses automatic or manual switching of detectors depending on the magnetic character of the substrate.
Accordingly, there is a need for a gauge probe for a combination coating thickness gauge for measuring both nonferrous coatings on ferrous substrates, using a Hall generator, and nonconductive coatings on conductive nonferrous substrates, using an eddy current search coil, that enables automatic switching of coating thickness gauging modes depending on an automatic determination of the nature of the underlying substrate. The gauge probe should contact the surface of the coated substrate in at most one place with a relatively small area of contact.